This invention relates to linear actuators and, more particularly, to an improved linear actuator of the type disclosed in copending U.S. application Ser. No. 579,432 filed on May 21, 1975 by Messrs. Halfhill and Brunner as a divisional of copending application Ser. No. 486,408 filed on July 8, 1974, now U.S. Pat. No. 3,922,718.
The unique linear actuator disclosed in copending applications Ser. Nos. 486,408 and 570,432 makes use of the principle that a roller frictionally engaged with the cylindrical surface of a drive shaft will be rotated about its axis by rotation of the drive shaft when such axis is parallel to the axis of the drive shaft, and will additionally be moved linearly in a direction parallel to the axis of the drive shaft when the roller axis is oblique to the axis of the drive shaft.
In general terms, therefore, the linear actuator disclosed in the aforesaid copending applications Ser. Nos. 486,408 and 579,432 includes a drive shaft having a cylindrical surface, means for rotatably mounting the drive shaft to a support frame for rotation of the drive shaft about its axis, means for rotating the drive shaft about its axis, a carriage to be driven along the predefined linear path between an inactive position and at least one active position, a roller, means for mounting the roller to the carriage with the roller being rotatable about a first axis and pivotable about a second axis perpendicular to the first axis, means for mounting the carriage to the support frame with the carriage being movable relative to the support frame along the predefined linear path between the inactive and at least one active position, and with the roller being in frictional engagement with the cylindrical surface of the drive shaft whereby the roller is caused to rotate about its first axis by rotation of the drive shaft when the first axis is parallel to the axis of the drive shaft and is additionally caused to move along the predefined linear path during rotation of the drive shaft when the first axis is oblique to the axis of the drive shaft, and means for controllably pivoting the roller about its second axis to control movement of the roller and thus the carriage along the predefined linear path.
As disclosed in the aforesaid copending applications Ser. Nos. 486,408 and 579,432, the linear actuator may be included in and form part of a magnetic disk drive. More specifically, disk drives generally include a drive spindle for rotating one or more magnetic recording disks. A head carriage is associated with each disk and may include two electromagnetic heads, one for each surface of the disk. Since information is recorded on the disk in concentric tracks which are spaced very closely adjacent one another, it is necessary to provide a linear actuator for the head carriage that is capable of moving the heads thereon to and from selected tracks on the disk at high speed and with great precision. Energization of the linear actuator to cause movement of the head-carriage assembly in the appropriate direction and speed is controlled by a suitable servo control system.
It is apparent that the precision and speed required in positioning the head-carriage assembly does not allow for slippage of the roller relative to the cylindrical surface of the drive shaft during such positioning. Accordingly, the roller is biased into engagement with the cylindrical surface with a requisite amount of force to prevent such slippage. During the normal operation of a disk drive, the heads may be retracted relative to the disks, i.e. spaced at a radius outside the outer-most track, for significant periods of time. Since, in the linear actuator disclosed in the above mentioned patent applications, the drive shaft is continuously rotation regardless of whether the heads are retracted or not, the roller and cylindrical surface of the drive shaft to which it is engaged will each by subjected to the same degree of wear whether or not the heads are retracted.